1.
Field of the Invention
The present invention generally relates to soft film biased (SFB) magnetoresistive (MR) sensors used for reading high density magnetic media and, more particularly, to an MR sensor design and fabrication methodology which provides significant Barkhausen noise reduction.
2. Description of the Prior Art
The use of SFB MR sensors to read high density magnetic media is old and well known in the art. SFB MR sensors are comprised of a magnetoresistive film with a soft magnetic film in close proximity thereto, where the magnetoresistive film and the soft magnetic film are separated by an insulative spacer layer. For example, U. S. Pat. No. 4,771,349 to Tsang discloses a magnetoresistive transducer which has a nickel-iron (NiFe or Permalloy.RTM.) magnetoresistive film, a tantalum (TA) spacer, and a nickel-iron-rhodium (NiFeRh) soft magnetic film. Electrodes connected to both ends of the magnetoresistive film allow an electric current to be passed through the magnetoresistive film. Current flow through the magnetoresistive film creates a magnetic field which magnetizes the soft magnetic film. The magnetized soft magnetic film produces a magnetic field which imparts a magnetic bias to the magnetoresistive film; hence, the name "soft film biased magnetoresistive" sensor.
The bias magnetic field serves to improve the sensitivity and linearity of the magnetoresistive effect. When the MR film is biased to its optimum state, the SFB MR sensor can receive a signal magnetic field produced by the recording media (tape or disc) and detect variations in the resistance of the MR film due to the polarity inversion of the signal magnetic field coming from the recording media. It has generally been assumed that a 45.degree. bias produces a bias state in the MR film where the variations of the resistance of the MR film produced by the polarity inversion of the signal magnetic field are symmetric with respect to the magnetic field. The prior art has described the optimum bias angle for SFB MR sensors to be 45.degree. with respect to the easy axis of magnetization of the MR film, and a 45.degree. bias has been associated with the lowest noise production. For example, U.S. Pat. No. 4,803,580 to Mowry discloses that it is old and well known in the art to bias the magnetization vector or current vector to 45.degree. to increase responsiveness to the angular change in the magnetization vector and to linearize sensor output. No studies have reported a relationship between the Barkhausen noise and the bias angle. Therefore, a need exists to establish the optimum bias angle which minimizes Barkhausen noise.
Until recently, the thickness and magnetic characteristics of the magnetoresistive film and soft magnetic film have not been given much consideration. U. S. Pat. No. 4,663,685 to Tsang discloses a magnetoresistive read transducer having a magnetoresistive NiFe film which is 200-500 Angstroms (.ANG.) thick and a soft magnetic NiFeRh film which is 100-500 .ANG. thick. U.S. Pat. No. 4,816,948 to Kamo et.al. discloses a magnetoresistive head where the thickness and magnetic characteristics of the magnetoresistive film and soft magnetic film are controlled. This patent states that the product of the saturated flux density of the soft film bias (SFB) layer and the thickness of the SFB layer is greater than 0.7 times the product of the saturated flux density of the magnetoresistive (MR) layer and the thickness of the MR layer. The relationship dictates that the product of the saturated magnetic flux density of the MR layer and its thickness divided by the product of the saturated magnetic flux density of the SFB layer and its thickness, which is commonly known as the magnetic ratio, shall be less than 1.428. The ratio establishes a 45.degree. bias or operating point for the MR sensor. The magnetostriction requirements necessary to minimize Barkhausen noise in an SFB MR sensor have not been discussed in the prior art.
The prior art suggests the use of a longitudinal field to suppress Barkhausen noise, and considerable effort has been devoted to methods of achieving the longitudinal field. These techniques require additional process steps and utilize space at the active surface of the recording head such that the placement of additional SFB MR sensor elements is constrained. Therefore, a need exists to avoid additional process complexity and to be able to space adjacent MR sensors in close proximity to each other.
The prior art has not addressed the special problems associated with the center-tapped or high density single ended MR sensors and Barkhausen noise suppression. For example, the idea of canted easy axis for MR noise suppression cannot be used with a differential center-tapped configuration.